Variation and Dip of the Magnetic Needle. 307 
bad observations, or were very much influenced by local attrac- 
tion. This chart can of course lay claim to no greater accuracy 
than that te which the individual observations are entitled; yet 
it has the advantage of presenting to the eye at one view a dis- 
tinct summary of. all the observations, and moreover it shows at 
a glance the approximate variation at places where observations 
have never been taken. It is of course desirable that the chart 
should be verified as extensively as may be, and if it should be 
the means of stimulating a single individual to undertake a series 
of accurate magnetic observations, taking care to present them to 
the public, the labor which this work has cost me will not have 
‘been expended altogether in vain. 
I had originally contemplated merely a collection of observa- 
tions on the variation of the needle. In the course of my in- 
quiries, however, I met with a few observations of the dip, and 
concluded to unite them all in the same article. All which I have 
been able to collect are contained in the following table. Those 
made by Professors Bache and Courtenay are from the 'Transac- 
tions of the American Philosophical Society, Vol. V. New Series ; 
those made at Cambridge are from the Memoirs of the American 
Academy, Vol. I. p. 68; those made by Sir John Franklin, Cap- 
tains Sabine and Back, are given in the Philosophical 'Transac- 
tions of London; the observation at Charlottesville was commu- 
nicated in a letter by Prof. Patterson ; those in Ohio were received 
from Prof. Locke, of Cincinnati; and the remainder are from 
Long’s Expedition to the Rocky Mountains in 1819 and 1820, 
and were furnished me by Mr, Herrick. _ - 
